1. Field of the Invention
The present invention relates to a light emitting device.
2. Description of the Related Art
Conventionally, light emitting devices using a technology that converts a light wavelength irradiated from a light emitting diode (LED) chip by a color conversion material have been developed. Among such light emitting devices, since a white LED is expected to be used for lighting, it is required for a while LED to have high-color rendering properties. Here, the high-color rendering properties refer to a property having a light emitting spectrum near sunlight. As the color rendering properties increase, the state of a color of an object becomes closer to a state of the color of the object which is seen under natural light In addition, it is also important to cause a light emitting device itself to be more efficient.
As a white LED, for example, it has been conventionally tried that a blue light emitting LED chip and a yellow light emitting conversion material emitting yellow light are combined so as to create white light to achieve high color rendering properties.
Recently, for the purpose of obtaining a white LED with further higher color rendering properties, a light emitting device as shown in FIG. 1 has been proposed. For example, according to a light emitting device 104 shown in FIG. 1, there has been proposed that an ultraviolet light emitting LED chip as a light emitting element 14, a blue light emitting color conversion material 34, which emits blue light, a green light emitting color conversion material 24C; which emits green light, and a red light emitting color conversion material 24R, which emits red light, are combined, so that white light is created to achieve further higher color rendering properties (see, for example, Japanese Patent Laid-pen No. 2000-31531).
As shown in FIG. 1, the light emitting device 104 described above disperses the blue light emitting color conversion material 34, the green light emitting color conversion material 24C; and the red light emitting color conversion material 24R in a transparent resin 54 so as to car out color conversion by using excitation light of the ultraviolet light emitting LED chip, which is the light emitting element 14.
Here, light emitting processes of the light emitting device 104 of FIG. 1 are shown in FIG. 2. As shown in FIG. 2, in the above-described light emitting device 104, the blue light emitting color conversion material 34 absorbs ultraviolet rays emitted from the light emitting element 14 as the excitation light to emit blue light. Specifically, electrons in the blue light emitting color conversion material 34 absorb the excitation light to transit from a grand state to an excitation state, and then emits blue light, which has a longer wavelength than that of ultraviolet rays, when returning to the grand state again. Similarly, the red light emitting color conversion material 24R absorbs ultraviolet rays as excitation light to emit red light In addition, the green light emitting color conversion material 24G absorbs ultraviolet rays as excitation light to emit green light In this manner, white light with high color rendering properties is created by using Pious kinds of color conversion materials.
However, since the above-described light emitting device 104 disperses the color conversion materials in the transparent resin 54, there is a problem in that light emitted by the color conversion materials may be absorbed again as excitation light of different color conversion materials.
For example, there may be a case where blue light emitted by the blue light emitting color conversion material 34 in the circumference of the light emitting element 14 shown in FIG. 1 is absorbed as excitation light by the green light emitting color conversion material 24G or the red light emitting color conversion material 24R before the light reaches outside.
In this case, as shown in FIG. 2, the blue light, which reaches at the green light emitting color conversion material 24G or the red light emitting color conversion material 24R, is absorbed as excitation light to emit green or red light
In this manner, light emitted from color conversion materials, which emit a short wavelength, has large energy, and is absorbed again by other color conversion materials, which results in an increase of conversion processes.
In addition, when compared with energy of excitation light absorbed by the color conversion materials, energy of light emission emitted from the color conversion materials is small.
As a result, there is a problem in that conversion efficiency of light emitted from the light emitting device 104 in relation to light emitted from the light emitting element 14 of the above-described light emitting device 104 becomes smaller. In particular, this problem is obvious in blue light, which has large energy and therefore is easily absorbed again by other color conversion materials.